Presently, automobiles produced for the United States market must be provided with a high-mounted, centered rear brake light in order to maximize the visibility of a caution indicator to drivers following the vehicle. The present state of the art uses a physical stoplight placed within the rearview window area, either on the rear dash surface or extending down from the roof of the car. This places the high-mounted stoplight in the optimum position for drivers following the vehicle; however, it also places a physical obstruction in the line of sight or viewing angle of the automobile's driver through his rearview mirror.
The configuration and position of the mounting of these high-mounted, centered brake lights also detract from the aesthetic design characteristics of the vehicle. Furthermore, many vehicles now offer spoilers or luggage racks as options. High-mounted stoplights are legally required to be visible from +10 degrees to -5 degrees in the vertical field behind the car. In most cases, the luggage rack or spoiler trunk mount options would not permit using dash mounted stoplights. One cannot simply move the stoplight higher up on the rear window, because the obstruction becomes more objectionable as it moves towards the center of the window. Placement of the stoplight at the top of the rear window or otherwise outside of the car becomes increasingly more expensive and aesthetically detracting.
A holographic rear window stoplight was disclosed in U.S. Pat. No. 4,892,369, and a "remotely illuminated transparent holographic stoplight for automobiles" was disclosed in U.S. Pat. No. 4,916,593. These holographic stoplights eliminate the physical centered stoplight from the rear window.
An optically transparent transmission hologram member is laminated onto the surface of the rear window. Therefore, it may be mounted high enough up on the rear window to allow diffracted light to clear a spoiler or luggage rack and, because of being transparent, the hologram is not an obstruction to the driver's rear view. The light source needed to activate the hologram is mounted in the rear deck below the rear window, and thereby out of the line of sight of the driver.
At certain times of the day there is a significant problem with holographic stoplights like that shown in U.S. Pat. No. 4,892,369. The sun, or any ambient source, will reach precisely the right angle to illuminate the hologram, be diffracted by that hologram toward the driver's rearview mirror, and enter and interfere with the driver's vision. This phenomenon is called ambient turn-on. Other sources will also be seen outside the vehicle which produce ambient turn-on.
Thus, in any environment where there is an intensive ambient source, a holographic film will be susceptible to ambient turn-on. This phenomenon is generally unwanted because the activation of the hologram should be controlled.